Ask Foghorn: What Barrel Length for a 300 AAC Blackout Rifle?

I get a lot of questions like this and figured that rather than responding to one specific email I should put it out there for other people as well. Reader Pete writes:

Hi Nick, I live in Amarillo and really enjoyed your article on your 300 BLK build. Appreciate so much your sharing of experience so that I don’t have to go through all the same trial and error. I have 2 Colt M-16s that I’m going to convert to 300 Blackout. Had a few questions for you if you’d be so kind as to give me a bit of advice . . .

1) Barrel length? I know that’s a long discussion, but I’m primarily wanting close range tactics with a very short barrel (likely 9″). Wondering if you know if there’s a significant advantage in range with a 10.5″ (both sub and super-sonic). It’s really hard to find data on accuracy down-range with different lengths and types (prob just go with AAC as they seem well respected, but would really like to find the most accurate nitrided barrel made). Would be really nice to be “accurate” (reliably hit 12″x12″ target at 200..maybe 250 yards?) despite the short barrel.

2) would you build again with the same silencer? (AAC) or have you seen / heard of any better?

So many thanks for your article and any advice you can give.

Cheers,
Pete

Let’s start with barrel length.

Every scrap of experience I have, and every bit of advice I’ve ever been given (with the exception of the black magic that is barrel harmonics), points to the conclusion that barrel length and accuracy have nothing whatsoever to do with each other. Longer barrels are not automatically more accurate. The only thing that barrel length gets you is velocity — the longer the bullet is in the barrel and being pushed by the expanding gasses of burning gunpowder, the faster that projectile will be going.

There is, however, a point of diminishing returns. Once all of the powder in the case has been burned, there’s no more fuel to add to the combustion and the gasses are at their maximum energy. Adding more barrel length after that point might still increase the velocity a bit, but the added weight isn’t always worth the extra dollop of velocity.

For 5.56 NATO, that point is 20″ of barrel. For 300 AAC Blackout, that point is 9 inches of barrel. That number comes from multiple conversations with the people who designed the 300 AAC Blackout cartridge, did the initial testing, and who continue to design guns around that caliber. On the flip side SIG SAUER seems to think that 6 inches is perfect for them (their new MCX uses a 6-inch barrel).

Nine inches of barrel will give you the perfect balance of light weight and velocity with the 300 BLK cartridge. It was designed to burn in short barrels and anything more is really just excess weight. The only real question is what twist rate to go with. Both AAC and Noveske use a 1:7 twist rate, while the PWS 300 BLK upper uses a 1:8 twist. The faster twist rate is perfect for the 220 grain ballistic monsters that subsonic 300 BLK ammunition use, but for the lighter hunting rounds (like Barnes’ 110gr round) not so much. Those rounds like a slower twist, something closer to 1:10, but no one seems interested in making a barrel for that. PWS is as close as you can get right now. Even so, 1:7 isn’t terrible.

As for silencers, there’s some competition out there.

Personally, if I had to do it all over again, I would definitely not go with a 762-SDN-6. There are lots of better options that have come on to the market in the last few years, and the 762-SDN-6 just hasn’t held up to the newer competition.

These days, #1 on my list, especially for new silencer owners, is the Liberty Mystic-X. The ability to shoot full auto 300 BLK through an SBR with this can attached is impressive. But what really makes me love it is the wide variety of calibers it can be used on. Everything from .22lr to subsonic .308 Winchester, and even 5.56 NATO from a 16″ barrel. And thanks to the mounting system, you can get multiple end caps to thread it on everything from 1/2 x 28 threaded barrels to H&K 3-lug mounts with a few seconds of twisting. I’ve soured on fast attach systems — not least because the ratchet on my 762-SDN-6 gave out after three years — and direct thread is the way to go. However, the stainless steel core will eventually wear out from too much full-auto fun.

For those looking for a rifle can and not worrying about rimfire or pistol calibers, the SilencerCo Harvester is perfect. Able to suppress all rifle calibers up to and including 300 Winchester Magnum, it’s perfectly at home on the 300 BLK rifle offering about 40% sound reduction. The end caps on this rifle can be changed as well to mount it on different rifles with different thread pitches, but the stainless steel baffles aren’t great for full auto fire (they will probably melt). For Pete this isn’t ideal, but for the average person this would be a good option.

The best option is one that you’re not going to like: wait. Rumor has it that SIG SAUER is coming out with their new line of rifle silencers at SHOT Show next year, and they are currently appear to be quieter and more durable than anything else on the market. And cheaper to boot. Stay tuned, because we will have more information on them as soon as that information becomes available.

In short, 9 inches is perfect for a 300 BLK barrel. As for silencers, either wait for the SIG SAUER cans to come on the market or get yourself an AAC MG-SD. While you can, that is.

Important correction Mr. Leghorn: the SilencerCo Harvester suppressor provides about 30 dB of suppression according to your graphic, not 40%. For you math types out there, a 30 dB reduction is enormous — actually better than some earmuffs if the number is correct.

Hey, man. Not a bad question at all. Basically, machine guns get exempt from the stamp for a shorter barrel. For an MG with a sub 16″ barrel and a suppressor, you only need two stamps, one for the suppressor and one for the MG.

“Adding more barrel length after that point might still increase the velocity a bit, but the added weight isn’t always worth the extra dollop of velocity.”

At this point, frictional (and other opposing) forces will start to DECREASE velocity. This is not a function of the dimensional cartridge at a top level so much as it is a function of the SPECIFIC loaded cartridge you are using. .300BLK may be great in a 9″ barrel with some loads (powder, charge weight, primer, bullet, internal case capacity, etc.), while others may be better in 8″ or 10″ barrels.

Consider a cartridge that has enough energy to travel 1000 yds in free air, standard conditions. Consider a rifle with a barrel 1000 yds long. The latter will stop the bullet long before it exits the barrel due to the additional forces of friction opposing opposing the bullets forward movement.

You can either start with a barrel length, or a baseline load. At this point, you should use internal ballistic calculation tools to match your selected variable to the other. Then, you can fine tune your load to the harmonics of the barrel you are using.

Also, the 300 BLK shoots faster from a 16″ barrel than a 9″ barrel, particularly with supersonic loads. I’m not sure if 18 or 20 or 24″ would give the absolute greatest velocity, as I haven’t seen the specs on 300 BLK velocity in a 24 inch barrel.

Velocity will increase as long as there is sufficient pressure behind the bullet to accelerate it. When barrel friction on the bullet is greater than gas pressure behind the bullet, the bullet will slow. That typically occurs in much longer barrels that are commonly in use. A 9 inch barrel is definitely a handy length, but it is certainly not the fastest.

A fast burning powder might fully burn in those 9 inch of barrel, but even so, the pressure at the end will still be way higher than it is outside the barrel – and hence the accelerating force on the bullet will still be more than friction. The powder needs not only fully burn, but also the products of said burning need to drop in pressure, before you start seeing a decrease in velocity. I don’t think you can get there in 9 inches with full-size powder loads regardless of how fast-burning they are.

Exactly what Int19h (?) said. Even the .357 magnum picks up considerable velocity when shot from a 16″ or 18″ carbine. Granted, revolvers have a barrel / cylinder gap, but that is a separate issue. The .44 Mag is well known for picking up considerable velocity from a carbine. The extra speed can be so great as to cause bullet failure.

We are not only talking about powder burn, but the pressure created by the expanding gasses of the powder. 5.56 pistols from 8 & 9″ barrels blow fireballs out of the barrel. My .460 XVR literally breathes fire. There is obvious powder burn outside the barrel which is clearly visible on video. It also singes the hair on my knuckles.

It’s the pressure of expanding gas behind the bullet that accelerates it, and it doesn’t fall off at 9″. Not even from a 300 BLK. It’s an efficient round, but probably gets max velocity from 18-24 or maybe even 26″ of barrel length. It is much less sensitive to muzzle velocity loss through short barrels than the 5.56.

A major benefit for me of using a 10″ barrel on my blackout is that I can run a 9 or 10″ handguard and still be able to use my can, that extra inch opens up a little bit more options in handguards. And yes, the .300 blk is designed to burn off in 9″, but its not guaranteed that every round you could possibly put through it will do so. An extra inch won’t hurt anything.

I would recommend the Silencerco Saker 7.62 instead. Much better QD system than AAC, and with the replaceable mounts, you can even go direct thread if you want. And the stellite core will be much more durable than the Harvester (and probably what Sig will come out with).

Great info Nick. I live vicariously thru you “new school” young guys, and your passion for AR platform and SBRs and various side uses- the hunting post you did awhile back was pretty useful, and points out the versatility of the .300.

I also note that this info is hard to find anywhere else, boiled down to “what works, for the average guy” based on a lot of industry insider info- and trying different things, and that is all you, Nick, and TTAG.

You remind me of that writer who was a big fan of the .270 back in the Field and Stream days.

Good follow up to your other posts on 300 black. I completely agree with the SDN-6, I have one too but like everything else, buying a can is a moment in time and you get what you get. It was state of the art just a few years back.

Here is the old barrel length again… Longer barrels have been proven to have better velocities and it depends on your burn rate on your powder whether you get the full use of your barrel. A 20 inch barrel will not hinder you with your AAC. The AAC uses a similar weight and dimensional bullet as a 30BR and 20 inch barrels and longer are the standard for those highly accurate bolt guns. A 30BR pushes from 107 grains to 120 grain 30 cal projectiles. The 30 BR is expected to shoot sub .5 MOA out to 300 meters and weight is at a premium for most of these guns. A lighter gun via barrel would allow more other things to be used. But shortening the guns usually has resulted in less accuracy.

You forgot one thing in your calculation – powder quantity. My .308 and my 300 BLK are both capable of pushing the same size bullet, but the .308 has way more space in the case for powder than does the 300 BLK. So in the case of .308, the longer barrel makes sense (at least 16 inches is needed to get full powder burn) because of the greater powder quantity. Since the 300 BLK stuffs a larger bullet into a cut down .223 case (which doesn’t hold all that much powder to begin with), you have less powder to play with and thus don’t need as much barrel.

I don’t know how the 30BR compares powder quantity wise to the 300 BLK, but that could account for the difference in barrel length.

A very hot load in 300BLK I use is 120gr Nosler BTHP with 21.0Gr H110. 21.5Gr shows pressure signs. Subsonic 220Gr loading is 9.6Gr IMR 4227 and that is a near capacity load – I’d guess there is room for 11Gr.

My test gun is a Remington 700 with 16″ 5R barrel. Velocities over my 9″ AAC barrel are surprisingly mild, about 30-40 FPS for subsonic and about 100 FPS for supersonics. There is no real difference in muzzle blast from either, both rifles use identical Surefire brakes.

Comparing my 5.56 barrels (18.5″, 14.5″ and 10.3″) is a whole different story.

That’s water weight capacity. Some powders should not fill the case because of their density and overpressure problems with that much powder. The difference between H4198 and TITEWAD would be a good example.

So, if a 5.56 requires 20 inches of barrel to burn all of its powder, would it make sense to reduce the powder quantity when shooting a 5.56 through a 16 inch barrel (or even something much shorter like a 5.56 pistol)? Seems a waste of powder if all it does is make a boom at the end.

It could, if you’re hand loading. Then you can also pick a slightly faster burning powder to get a complete burn in 16″ of barrel. Military manufacturers will still load for 20″, because they have to accommodate the M4, M16, M249, etc.

A big boom, and a really big flash, sounds like fun, when fun is the goal, anyway. So would that be what they’re doing to make that so-called “dragon’s breath” novelty ammo, just adding a lot of extra powder to burn beyond the muzzle? Or is it a different type of powder altogether? I’ve only ever seen youtube videos of it. Looks wild.

While I can see Nick’s point in preferring screw on suppressors over fast attach systems, there are reasons why the fast attach is superior in some cases. First of all, with 300 BLK, if you are switching from supersonic to subsonic rounds, you might want to be able to quickly make the change from unsuppressed to suppressed. Unscrewing a muzzle device and replacing it with a suppressor in the field sucks. Been there, done that. Sure, you could just leave the suppressor attached, but I’d rather not have it see the wear and tear if it doesn’t need to.

Secondly (and the reason why my second .30 suppressor is fast attach) is that I have multiple firearms that I take to the range and want to be able to quickly switch between them. A fast attach lets me do that.

As for waiting on Sig Suppressors – good luck with that. They announced their current suppressor line at the 2013 shot show and we are now just starting to see them trickle into the retail channel. Any 2015 announcement would probably not hit shelves until 2017 and then figure the ATF wait and you are probably going to need to wait until 2018 to get your suppressor.

In my ignorance, I would say its because I want to reduce wear and tear on the suppressor. I assume that there is a finite lifespan of the suppressor guts and shooting supersonic ammo at the range kind of defeats part of the suppressor benefit. In a tactical situation, I can see the reason for keeping it attached, but at the range, supersonic ammo is not going to be ear safe with or without a suppressor.

I would look into making/getting a telescopic suppressor (“reflex” design). That way you could further save length on the rifle. Having the barrel + suppressor be only 9+4 inches (13) long would be neato.

I was thinking something along those lines, too. Maybe not so much that the longer barrel is directly responsible for greater accuracy, but indirectly? Bullet drop is calculable, though, so that should really fall under marksmanship, as opposed to intrinsic accuracy of the rifle? Not sure, and maybe I’m just splitting hairs.

If that’s a factor, then it would maybe only come into play at great distances? That still wouldn’t explain people’s prevailing view of barrel length driving accuracy, since the majority of shooting is not done at extreme distances. So, I don’t know, but it’s all very interesting to me.

The point is, based on the design parameters of the 300BLK round while you do get more velocity from a longer barrel the % increased velocity per inch increase in barrel isnt as much as with say a 5.56. There was an article on TTAG maybe a year or so ago comparing velocity loss from from a 5.56 16″ vs 10″ and a 300BLK from 16″ to 9″. Longer barrels also screw up the timing, 16″ with carbine gas is on the outside fringe of reliable for the 300. By design, the 300BLK just doesnt give up that much velocity until you get down below 9-10″ of barrel using factory ammo. Play around with powders too much handloading and you screw up the timing on the gas system. Pretty much the only reason for a 16″ 300BLK is if you don’t ever plan on running a suppressor and don’t want to register as an SBR. In a 16″ supersonic only AR there are vastly superior caliber choices like 6.8SPC. 300BLK supers wont touch the 6.8SPC in a carbine length or longer barrel, and if you dont plan to suppress or run an SBR is just a way better choice. In barrels16″ or longer it is going to have a good 100-200 fps over the BLK using 110-115 grain projectiles.

The Harvester is a great suppressor. Lots of reasons for one to get get launched down range, but most have one thing in common, HUMAN ERROR. It probably worked loose after multiple shots, and somebody forgot to tighten it back up. I’ve seen a few other suppressors suffer the same fate, for the same reason. That’s the one, and only reason I’m so anal about checking mine on the range, or in the field.

Different calibers reach their highest velocity at different barrel lengths. For .22LR, for instance, the optimum length is 16”. Any length over that will actually result in lower velocities. Pistol calibers often reach near optimum velocity at about 8-10”; longer lengths may increase velocity, but only by slight amounts.

Higher velocity results in less drop at longer ranges, which of course effects point of impact. However, even worse for accuracy is the projectile slowing from supersonic to subsonic velocity. Passing through the sound barrier imparts wobble. (In the early days of jet aircraft, a lot of planes were shaken apart when passing through the sound barrier.) Therefore, long range .22LR shooters stick to subsonic ammo in order to avoid the sound barrier transition. Drop is predictable; wobble is not.

Longer barrels can be more accurate when using fixed sights, because a longer sight radius allows for finer adjustment of aim. However, heat build up effects point of impact. If you’ve ever noticed, experienced shooters allow their rifles to cool between shots, to avoid changes in POI. This is why target shooters prefer short fat barrels — they distort less under heat and cool faster. Their POI remains more consistent.

ANd then you have barrel harmonics to contend with. The longer the barrel, the greater the tendency for it to move while firing imparting a POA-POI shift. This is one reason bench rest guns all have super heavy barrels. The stiffer barrel damps harmonics. On a gun you plan to carry, super heavy barrels are not ideal.

The disadvantage with direct threading is that it will loosen up and start backing off while shooting. QA/QD systems may add more tolerance stacking and complexity, but the silencer is snug and firm. The last thing you ever want is a loosening silencer while shooting rapid fire or full auto.

A more rigid barrel is better. Longer barrels tend to be less rigid, so people go for bull barrels, but those get heavy so people go for fluted barrels. A faster bullet requires a different twist to stabilize I think, and if the twist is too fast the bullet will sail like a kite instead of soaring, but it’s true the faster bullet will be flatter trajectory, and flatter can feel like accuracy for most people who don’t want to adjust for elevation. A bullet also travels less horizontal distances from wind when it is faster. It gets pretty messy from there.

I have a 762-SDN-6 suppressor on a 300 BLK rifle I built with a 16″ barrel shooting subsonic 220gr ammo but I don’t get the tick, tick, tick when shooting. I get a rifle firing sound which seems down range. Would a shorter barrel make it quieter?

Mike: yes. So would a better/different suppressor. The long Sig Titanium 7,62 is REALLY quiet. Just a tiny bit louder but shorter and lighter is the versatile Silencerco Omega. That is your shortest, lightest choice that will give you a tiny snap snap like a loud clap…around 119-121 db. I have seen some measurements as high as 123db which is still VERY quiet. The SDN just doesnt silence 300 Black very well. Your longer barrel does add minimal velocity. There is almost NO need to have a barrel longer than 8-9″.
Hope this helps.

Mike: I cant seem to edit my comment. I need you to read this test by The SIlencer Shop that shows the SDN can at an enormous disadvantage when shot with 300 BO subs. A measurement of 130db is horrible. Compare that to the Omega at 120db and you have a HUGE audible difference.
Here is the link. I hope it will post.http://blog.silencershop.com/300-blk-impressions/
Sorry to be bearer of bad news. But on the good side, you cant have too MANY cans. I recommend the Silencerco Omega FTW.

Hey Nick. Sig ONLY uses that 6.75″ (not 6″) barrel length for their Military full auto Black Mamba entry gun MCX. They designed it for the needs of Special Forces to be real compact (as an Mp-5) and QUIETER than an MP-5. It has a very short barrel and runs a long 9″ can. And it is VERY quiet AND compact. Mission accomplished.
Realize that this gun is SPECIFICALLY designed for ultra close covert CQB use and is not concerned with supers and 100% powder burn.

It is a niche gun not even available to the public. You would have to cut down and make your OWN 6.75″ barrel since SIg will not sell you one of theirs.
Currently they offer a 16″ 300 Blackout barrel for their MCX. I think you still have cut the barrel down if you want a shorter barrel.
Sig does plan for a civilian 300 barrel. It will be NINE inches in length. I dont think they are shipping as of yet…which is a big question why. But there you have it.

I want to use ‘both’ sub and supersonic ammunition and am a reloader so leaning toward the 1/8 twist to take advantage of the calibures flexibility. I do anticipate hunting deer / pig with this build supersonic of course. It will also be fitted with an Omega can. Any insight on following:
– Wilson Combat 11.3″ barrel (worried this is too long)
– Lantac or Gemtech BCG (want to ensure proper cycle)
– Adjustible Gas Block???

To add I’m also a lefty so want to manage the gas as much as can be done. TKS!

What happened to the big appeal of the 300 AAC for the average guy when this round was first released, which was “you can put it on any AR-15 upper you have and go shooting”?? – What happened to ‘it will fire from any AR-15 you might have” – doesn’t appear to be the case since now you say a 9″ barrel is optimal but which would require an SBR $200 stamp, you may need an adjustable gas block to handle over gassing & shoot subsonic and then there’s the ammo is twice the cost what I pay for my 556 rounds.. My interest in building a 300 AAC for hog hunting that I could just slap in my upper is quickly deteriorating, especially since the gun may not even cycle without, additional hardware if you want to what was promised, which is shoot both supersonic and supersonic through a stock 16″ barrel.

Hi Lance, well I thought like you did, stick it on and go but being a dumb old guy I did not bother to research much. I’ve loaded 5.56 for over 30 years. Have had a LOT of different AR’s including a Colt upper in 7.62×39 that I killed my first deer with! I had a table at the local gun shows for years. So anyway, as I said, I did not bother to do anything but read about reloading data……knew nothing about “over or under gassing” or any of the other weird things. I just ordered a 16 inch barrel and stuck it on my Stag upper. I screwed my “walking dead”, LEGAL, form 1 Maglite silencer on it. I make my living building high end audio equipment, I can’t afford “ear damage”. It is ear safe with subs and damn near it with supersonics.

The thing functions with everything……from 125 grain Sierra with 17 grains of Lil’ Gun to 220 grain Hornady with 10.5 of AA 1680. With or without the can. Accuracy is hard to figure…….I have a real 25 yd range and with the red dot thing on it it stuffs all loads into a ragged hole but the point of impact does vary between them.

Speared on by no issues I built a 10.5 inch “pistol” with an “arm brace” LOL. With the can on it, it also is ear safe and the 25yd accuracy is like the other one with the same type red dot. It too functions with all loads I have. I’ve not shot it w/o the can.

A buddy has a 100yd range in his back yard. I took that pistol with 220 grain ELD-X over 10.6 grains of AA 1680 And hit the 19″ plate the second shot with the red dot! I was amazed at myself. I then hit it 15 times in a row.

And now for the killer………..I dumped ALL my 5.56 stuff, all of it including the progressive press set up just for it, and went full in on 300 BLK. I like loading 300blk with a hand press for some unknown reason…..I developed an affinity for the hand press when I was loading for my 460 XVR. Just more satisfying to me than a “real press’. I did 100 rounds a night for 6 nights. Damn was my arm sore on the 7th day, so I rested 🙂

So, don’t read to much into the issues. My guess is your luck will be like mine. or you’ll solve them. If you look up brown recluse or rattlesnake bites on the internet you’ll be scared shitless from the images. I’ve been bitten by both. Nobody shows pics like mine because it was not big deal……nothing to talk about. I think “problems” with any firearms are possible and the worst will show up on the internet (like snake and spider bites) because folks need help but it’s probably the exception not the rule!

Just an old farts opinion!
Ed
ps…….the “arm brace” apparently was deemed legal by ATF for “incidental resting on your shoulder”. I reckon most of us can read between the lines on that.